1. Field of the Invention
The field of this invention generally relates to footwear, and more particularly to an article of footwear having a fluid-filled resilient cushioning device in a sole that provides dynamic cushioning and support for the comfort of the wearer.
2. Background of the Invention
One of the problems associated with footwear, especially athletic shoes, has always been striking a balance between support and cushioning. Throughout the course of an average day, the feet and legs of an individual are subjected to substantial impact forces. Running, jumping, walking, and even standing exert forces upon the feet and legs of an individual which can lead to soreness, fatigue, and injury.
The human foot is a complex and remarkable piece of machinery, capable of withstanding and dissipating many impact forces. The natural padding of fat at the heel and forefoot, as well as the flexibility of the arch, help to cushion the foot. An athlete's stride is partly the result of energy which is stored in the flexible tissues of the foot. For example, a typical gait cycle for running or walking begins with a “heelstrike” and ends with a “toe-off”. During the gait cycle, the main distribution of forces on the foot begins adjacent to the lateral side of the heel (outside of the foot) during the “heel strike” phase of the gait, then moves toward the center axis of the foot in the arch area, and then moves to the medial side of the forefoot area (inside of the foot) during “toe-off”. During a typical walking or running stride, the Achilles tendon and the arch stretch and contract, storing and releasing energy in the tendons and ligaments. Rolling the foot forward through the step from the heelstrike to the toe-off releases the energy stored in the Achilles tendon and arch, which helps to propel the foot into the toe-off.
Although the human foot possesses natural cushioning and rebounding characteristics, the foot is greatly assisted in effectively overcoming many of the forces encountered during athletic activity through the use of appropriate footwear. Unless an individual utilizes footwear which provides proper cushioning and support, the soreness and fatigue associated with athletic activity is more acute, and its onset is accelerated. The discomfort for the wearer that results may diminish the incentive for further athletic activity. Equally important, inadequately cushioned footwear can lead to injuries such as blisters; muscle, tendon and ligament damage; and bone stress fractures. Improper footwear can also lead to other ailments, including back pain.
Proper footwear should complement the natural functionality of the foot, in part by incorporating a sole (typically including an outsole, midsole and insole) which absorbs shocks. However, the sole should also possess enough resiliency to prevent the sole from being “mushy” or “collapsing,” thereby unduly draining the energy of the wearer. Ideally, the footwear would also mechanically assist the foot through the step by releasing stored energy simultaneously to the release of energy stored within the Achilles tendon and the arch, thereby contributing to the springiness of the step.
In light of the above, numerous attempts have been made to incorporate into a shoe improved cushioning and resiliency. For example, attempts have been made to enhance the natural elasticity and energy return of the foot by providing shoes with soles which store energy during compression and return energy during expansion. These attempts have included the formation of shoe soles that include springs, gels or foams such as ethylene vinyl acetate (EVA) or polyurethane (PU). However, all of these tend to either break down over time or do not provide adequate cushioning characteristics.
Another concept practiced in the footwear industry to improve cushioning and energy return has been the use of fluid-filled systems within shoes soles. The basic concept of these devices is to have cushions containing pressurized fluid disposed adjacent the heel and/or the forefoot regions of a shoe.
A particular area in need of cushioning is the heel region. As noted above, when running or walking in a typical fashion, the heel region of the foot or shoe strikes the ground first, bearing the full brunt of the impact of the step. A cushioning system is needed that will absorb the forces of the heelstrike, while simultaneously assisting the wearer to propel the foot forward through the rest of the step.